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A newly wed Auckland couple have been left flummoxed after excitedly awaiting their marriage certificate - only to receive someone else's.

The couple, who only want to be known as Emma and Michael, say they had a great wedding in the city of the sails on February 4 and duly sent off the required monies to get their marriage certificate.

However, Emma says she was surprised to find a certificate belonging to another couple.

"It's pretty shocking really. I just want to know where our marriage certificate is. I'm an honest person and I'm not going to do anything with that information but how can they guarantee ... who else isn't getting sent a whole bunch of other stuff."

The Department of Internal Affairs has since apologised to the couples involved and said it was the result of "human error".

Emma said there was a plethora of private information available on the certificate that a dishonest person could have a field day with.

"I know their full name, their address, I know both of their date of births, I know their parents' names, I know their parents' maiden names and where they live at the moment and where they were born. So yeah, I know a lot.

"There's a lot of information on there that I could use if I was not an honest person, potentially use for identity theft and things like that. The concerning thing for me is where is ours and how can they guarantee to me that my certificate hasn't ended up in the hands of someone that will commit something like that."

She also wanted assurance that people's personal information was being treated "with the care that it needs to be treated".

Emma eventually went on to Facebook and found the couple, who weren't sure what to make of her approach.

That couple, who are also from Auckland, say they feel both "upset and disappointed" that their "private information has been so carelessly given out to somebody else that we don't know at all".

"We were quite shocked and there was a general disbelief when Emma contacted us about her receiving our marriage certificate. It's not like there was a mix up and we both received each other's certificates, but rather two copies of our certificate were sent to both us and Emma and her husband.

"If Emma had not been so courteous and forthcoming, then who knows what would have happened with our information. We're not even sure if births, deaths and marriages would have picked this up on their own."

She said they had lost faith in the department.

"Right now we are quite upset with the Department of Internal Affairs and we feel a general lack of trust with future requests from them."

She said they had received an apology but questions about what was being done to ensure it wouldn't happen again went unanswered.

''She just kept re-emphasising the point that they have a 99.9 per cent success rate and that this 'doesn't happen often'.

"Again, we have received an apology, but the damage has already been done and we feel like there isn't really anything else that could be done or said to make up for it."

In a statement, Registrar General of Births, Deaths and Marriages Jeff Montgomery said the department issued around 45,000 marriage certificates a year and "have a 99.9 per cent accuracy rate but unfortunately in this instance we got it wrong".

"The process although largely electronic does have a human element and this is where the error occurred. Incorrect data entry meant the couple's marriage certificate was created but never issued and they ended up receiving the certificate ordered by another customer.

"We understand the importance of this role and do have quality assurance processes in place. The department is extremely sorry for the error and has apologised to the customer concerned.

"Our staff are also very upset that this has happened and have been reminded of the importance of accurate data entry."

Montgomery said Emma and Michael's certificate was put in the courier on Tuesday.

Emma was unimpressed by the department's response.

"It's still not good enough really and who's to say this hasn't happened a lot more than they realise. I'd love to know where the 99.9 per cent accuracy comes from and how they qualify that."